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Nov. 4, 2011 -- The FDA has approved Janssen's Xarelto to prevent stroke in patients with an abnormal heart rhythm called atrial fibrillation -- with a strong "black box" warning.

A black box warning is the FDA's strongest warning.

Xarelto had previously been approved to prevent blood clots in patients receiving hip and knee replacements.

Xarelto now joins Boehringer Ingelheim's Pradaxa as alternatives to warfarin (brand names include Coumadin and Jantoven) to prevent stroke-causing blood clots in patients with atrial fibrillation not caused by a heart valve problem. Atrial fibrillation often puts a person at increased risk of blood clots and stroke.

Like warfarin, Xarelto is a blood thinner -- an anti-clotting drug. These products can cause dangerous bleeding. But Xarelto can also increase the risk of stroke if people stop taking it without medical supervision. That's the main warning in the "black box" on the Xarelto label.